Is it okay to say kiddush or havdalah twice in order to be Motzi someone else (e.g. wife)?
Like most halacic issues, it's a machlokes. See Shulchan Aruch 273:4 who allows this only if the other cannot do so himself. The Mishna Berurah (20) brings the Pri Chadash who allows this in any case, while the Artzos haChayim allows it in any case, but does not consider it preferable if the other can do so him/herself.
(The Aruch haShulchan in OC 273:5 gives the reasoning as Kol Yisrael Areivim Ze baZe. This applies only to birchos mitzva and not other brachos.)
In Berachos 29a the Gemara says that a person may make a Bracha for others even if he himself has already fulfilled his obligation. The Rosh states that based on this Gemara a person may make Kiddush for his family members even if he made it previously.
There are two components of Havdalah. The first is the Havdalah we say during prayer, and the second is Havdalah over a cup of wine.
There are two opinions as to the source of saying Havdalah during prayer. One opinion is that Havdalah is from the Torah (Zachor es yom hashabas lekadsho, Remember the Shabbos to keep it holy, which one must do when shabbos comes in and goes out). A second opinion is that Havdalah is a Rabbinic enactment, and the verse "Zachor es yom hashabas lekadsho" is only for Kiddush.
According to the opinion that the Havdalah we say during prayer is obligatory by Torah law, Havdalah over wine is a Rabbinic enactment connected to the Havdalah during prayer. Therefore, all who are obligated in Havdalah during prayer (men and women) must say Havdalah over wine.
According to the opinion that the Havdalah during prayer is Rabbinic, there are two reasons given for Havdalah over wine. Some say it's an independent decree to differentiate between the holy and the secular, and not connected to Zachor at all. Therefore, women have no obligation to say Havdalah over wine. Others say the decree is an extension of Zachor, which means women would be obligated in Havdalah over wine.
So, once a man did Havdalah, he should not do it for women because according to the second opinion, women are not obligated, and if there is a doubt over whether to say blessings or not, one doesn't. Women can make Havdalah for themselves, however, like they can make a blessing over any commandment in which they aren't obligated.
Source: Shulchan Aruch Harav 296:19
On your last paragraph, that is true, except that Sepharadim don't allow women to make a Berachah if they aren't obligated.– Seth JSep 20, 2011 at 19:42
There are also Kabbalistic reasons for women not to make their own Havdalah, as well as for them not to drink the wine (or so I'm told; I've never seen a source, though).– Seth JSep 21, 2011 at 13:49
Mishna Berura covers this exact scenario regarding havadalah in O.C. 296:8 s.k. 36. He says someone who has already made havdalah, or already heard havdalah, should not make it again on behalf of ladies unless there is at least a child there to listen (seemingly from the standpoint of chinuk), because one opinion holds that women aren't obligated in havdalah and the man would thus be making a bracha l'vatala. He brings another opion which allows l'chatchila, but concludes that if she's able to make havadalah herself, why should the man enter into a situation of sofeik.
1Welcome to Mi Yodeya. Nice answer. Hope to see you around the site.– mevaqeshOct 25, 2015 at 1:21